1. Field of the Invention
The present invention relates to a low pressure exhaust gas recirculation (EGR) apparatus.
2. Description of Related Art
A known low pressure EGR apparatus recirculates a portion of exhaust gas of an internal combustion engine from a low exhaust gas pressure area of an exhaust passage (a low exhaust gas pressure generating area, such as an area on a downstream side of a diesel particulate filter, which will be hereinafter abbreviated as DPF) to a low negative intake air pressure generating area of an intake air passage (a low negative intake air pressure generating area, such as an area on an upstream side of a throttle valve).
Previously proposed techniques will be described with reference to FIGS. 10 and 11. In the following description, similar components are indicated by the same reference numerals.
With reference to FIG. 10, a high pressure EGR apparatus 231 limits generation of NOx (nitride oxide) in exhaust gas of an engine 202. The high pressure EGR apparatus 231 is also often simply referred to as an EGR apparatus. The high pressure EGR apparatus 231 recirculates a portion of exhaust gas, which flows through an exhaust passage 203, as EGR gas to an area (a high negative intake air pressure generating area) on a downstream side of a throttle valve 225 in an intake air passage 204. With this construction, the EGR gas can be mixed into the intake air to limit the combustion temperature in a combustion chamber of the engine 202 and thereby to effectively limit the generation of NOx.
In the high pressure EGR apparatus 231, a high pressure EGR regulating valve 233 is provided in a high pressure EGR passage 232, which recirculates the EGR gas to the intake air passage 204. The high pressure EGR regulating valve 233 regulates an opening degree of the high pressure EGR passage 232. An engine control unit (ECU) controls an opening degree of the high pressure EGR regulating valve 233 such that a corresponding EGR quantity (a quantity of the recirculated exhaust gas per unit time), which corresponds to an operational state of the engine 202 (e.g., an engine rotational speed, an engine load), is obtained.
Furthermore, there is a constant market demand for a technique that further reduces the generation of NOx at the engine 202.
For instance, Japanese Unexamined Patent Publication No. 2008-150955A (US2008/0141671A1) teaches a technique of a low pressure EGR apparatus, which is provided in addition to the high pressure EGR apparatus, to reduce the generation of NOx.
One previously proposed technique, which employs the low pressure EGR apparatus, will now be described with reference to FIG. 11. The low pressure EGR apparatus 201 is an apparatus, which recirculates a portion of the exhaust gas from a low exhaust gas pressure area of the exhaust passage 203 (an area of the exhaust passage 203, which is located on the downstream side of the DPF 228 in the exhaust gas flow direction and at which the low exhaust gas pressure is generated) to a low negative intake air pressure generating area of the intake air passage 204 (an area of the intake air passage 204, which is located on the upstream side of the throttle valve 225 in the intake air flow direction and at which the low negative intake air pressure is generated). Thereby, the low pressure EGR apparatus 201 is adapted to recirculate the small quantity of the EGR gas to the intake air passage 204 with the relatively high accuracy.
Specifically, for instance, the low pressure EGR apparatus 201 of the vehicle, which has a turbocharger, recirculates the EGR gas from the area of the exhaust passage 203, which is located on the downstream side of the DPF 228 in the exhaust gas flow direction, to the area of the intake air passage 204, which is located on an upstream side of a compressor 223 in the intake air flow direction. By recirculating the exhaust gas from the low exhaust gas pressure area of the exhaust passage 203 to the low negative intake air pressure generating area of the intake air passage 204, it is possible to recirculate the small quantity of the EGR gas to the engine 202.
Therefore, although it is difficult for the high pressure EGR apparatus 231 to limit the generation of NOx in the operational range of the engine, at which the low concentration EGR gas is required, such as the operational range of the engine, at which the engine load is large, the low pressure EGR apparatus 201 can limit the generation of NOx even in the operational range of the engine, at which the low concentration EGR gas is required.
In the low pressure EGR apparatus 201, a low pressure EGR regulating valve 206 is provided in a low pressure EGR passage 205, through which the EGR gas is recirculated from the exhaust passage 203 to the intake air passage 204, to regulate an opening degree of the low pressure EGR passage 205. Similar to the high pressure EGR regulating valve 233 described above, the opening degree of the low pressure EGR regulating valve 206 is controlled by the ECU to provide the EGR quantity, which corresponds to the operational state of the engine 202 (e.g., the engine rotational speed, the engine load).
The low pressure EGR apparatus 201 recirculates the portion of the exhaust gas from the low exhaust gas pressure area of the exhaust passage 203 to the low negative intake air pressure generating area of the intake air passage 204.
Therefore, although the low pressure EGR apparatus 201 can be effectively used to recirculate the small quantity of the EGR gas to the engine 202, it is difficult to recirculate a large quantity of the EGR gas to the engine 202 through use of the low pressure EGR apparatus 201. That is, even though there is the operational range of the engine 202, at which the large quantity of the EGR gas needs to be recirculated to the engine 202, the low pressure EGR apparatus 201 cannot be used to provide the large quantity of the EGR gas to the engine 202.
It is conceivable to provide an intake air throttle valve 207 (a negative intake air pressure generating valve), which can generate a negative intake air pressure, in the intake air passage 204, to which the low pressure EGR apparatus 201 recirculates the EGR gas. In the operational range of the engine 202, in which the large quantity of the EGR gas should be recirculated to the engine 202, the intake air throttle valve 207 of the low pressure EGR apparatus 201 may possibly be controlled in the valve closing direction thereof (the direction for generating the negative intake air pressure). That is, in the operational range of the engine 202, in which the large EGR quantity should be recirculated to the engine 202, the negative intake air pressure may be generated through the use of the intake air throttle valve 207 to recirculate the large quantity of the EGR gas.
However, as recited above, the opening degree of the low pressure EGR regulating valve 206 is controlled according to the engine rotational speed or the engine load.
The intake air throttle valve 207 is controlled to the valve closing direction only in the operational range, in which the large EGR quantity is demanded by the ECU.
As discussed above, the low pressure EGR regulating valve 206 and the intake air throttle valve 207 are controlled based on the different operational factors, respectively. Therefore, the low pressure EGR regulating valve 206 and the intake air throttle valve 207 are independently operated.
Thus, a dedicated actuator J1, which drives the low pressure EGR regulating valve 206, and a dedicated actuator J2, which drives the intake air throttle valve 207, are required. Therefore, this will possibly result in the cost increase, the size increase and the weight increase.
Thus, in order to reduce the size, the weight and the costs, it has been demanded to drive the low pressure EGR regulating valve 206 and the intake air throttle valve 207 with a single electric actuator (a drive means using, for example, an electric motor).
Therefore, it has been proposed to provide a single electric actuator to drive the low pressure EGR regulating valve 206 and to transmit the output of the single electric actuator to the intake air throttle valve 207 through a link device (a drive force transmitting mechanism).
In such a case, the link device may include a converting mechanism, such as a cam groove, which converts the output (output characteristic) of the electric actuator and transmits the converted output (output characteristic) to the intake air throttle valve 207. In this way, when the opening degree of the low pressure EGR regulating valve 206 becomes larger than a predetermined opening degree, the opening degree of the intake air throttle valve 207 can be reduced (i.e., the negative pressure can be increased) synchronously with the increasing of the opening degree of the low pressure EGR regulating valve 206.
However, in the case where the mechanism of transmitting the output of the electric actuator to the intake air throttle valve 207 through the link device, it is necessary to enable diagnosing of a failure of the intake air throttle valve 207 at the time of occurrence of the failure of the intake air throttle valve 207 caused by a malfunction of the link device (e.g., a malfunction of unintentional disconnection of a cam plate, a malfunction of disengagement of the link).
In view of the above need, it is conceivable to provide an independent valve opening degree sensor, which senses the opening degree of the intake air throttle valve 207 separately from the low pressure EGR regulating valve 206 to determine the failure of the intake air throttle valve 207.
However, the possibility of occurrence of the failure of the intake air throttle valve 207 is very small. Therefore, the provision of the dedicated opening degree sensor to the intake air throttle valve 207 causes a disadvantageous increase in the manufacturing costs, so that the advantage of the cost-effectiveness becomes very low.